Current



June 10, 1930. J. J. GILBERT 1,763,042

' LOADED SIGNALING CONDUCTOR Filed June 6, 1928 500 I000 I500 200p g. fie. 3.

O .05 .IO .15 0 .05 Y J0 J5 CURRENT fi Cu/ms/vr il lllll lll l =lll l l l l lIQ fie. Z

//VVN7'0R. JOHN J GILBERT On a telegraph cable operated duplex these variations in the characteristics of a loaded cable become particularly objection able because it is difficut and expensive to build an artificial line that will have the same variable resistance and inductance characteristics as'the loaded cable. On a telephone conductor impedance variations with the current variations are objectionable because they produce intermodulation of the differentfrequency waves comprising the speech spectrum, or different speech channels, with resultant distortion of the speech.

In general, the objectionable effects resulting from loading only become serious when the current is large, for, as shownby Fig. 1, the variations in resistance and inductance are not material for variations in current from zero to less than about .02 ampere. Therefore, in the case of a long conductor (by long is meant a conductor not necessarily long geographically but one having a great overall attenuation) objectionable effects resulting from loading would originate on the terminal sections only of a conductor becau'se'the transmitted currents are rapidly attenuated and, after traversing a length of conductor which may be a relatively small fraction of the total length, they are reduced to such values that the variations in impedance resulting therefrom become negligibly small. The relative magnitudes of the currents in a typical submarine cable at various points along its length are shown in the curve of Fig. '2. This curve indicates that the current is reduced to less than half its original amplitude at a point about 200 nautical miles from the terminal and that at a distance of 500 miles the current is reduced to a value at which the variations in the resistance and inductance of the loaded con ductor may be neglected. I

Since it is in terminalportions only of a loaded cable thatthe objectionable charcteristics of the loading arise, those portions of the conductor may be modified by making suchchanges as are necessary in the loading to reduce'the variations in impedance without altering the normal loading on the main portion of the cable. It would be undesirable because of cost to construct a cable which employs throughout its length a conductor of the specially loaded types which will be described for useon the terminal portions.

There are several methods of reducing the impedance variations on the terminal sections. The simplest of course is to eliminate all the loading material and this may be desirable in some instances. This method has the disadvantage, however, that it in.- creases the attenuation of the terminal section and causes reflections at the junction with the main section due to the abrupt change in inductance at that point.

The preferred method in accordance with this invention is to so change the quantity and quality of the loading material on the terminal sections that the impedance variations due to the loading may be considerably reduced Without reducing materially the effective inductance and transmission efliciency of the conductor. It is possible to obtain such a result by increasing the thickness of the loading and reducing its permeability;

This is so because, in general, magnetic materials of low permeabilities have fairly constant permeabilities over a greater range of magnetizing forces than do higher permeability materials, particularly over the range of magnetizing forces to which continuous loading on a submarine cable is exposed. There are some exceptions to this general rule but it holds to this extent at least, that from thevarious'known magnetic materials it is feasible to select materials of more constant permeability among those of low permeability than among those of higher permeability. Since the inductance of a loaded conductor depends largely on the amount and permeability of the loading material it follows that, other things being equal, a conductor loaded with magnetic material oflow permeability will have a more constant, though lower, inductance than one loaded with the sameamount of high permeability material, and by increasing the amount of magnetic material on the first conductor its average inductance may be increased to that of the second conductor without increasing the variations in the inductance'to the same extent.

The use of a greater thickness of lower permeability loading material, in addition to making the, inductance more constant, decreases variations in the effective resistance of the conductor by reducing variations in the hysteresis and eddy current losses.

Variations in hysteresis losses are less be cause at the magnetizing forces with which we are concerned hysteresis loss varies as the square or a higher power of the flux density. It follows that since the flux density in the loading material is a direct function of its permeability, reducing the permeability of, and increasing the thickness of the loading material in inverse proportion thereto, will reduce the maximum hysteresis loss (as caused by the, maximum magnetizing force) to a greater extent than it Will the minimum hysteresis loss. Therefore the variations in hysteresis loss over the range of magnetizing forces will be less.

Variations in the eddy current losses are less because these losses also vary as the square of the flux density. Furthermore it is possible to greatly increase the resistivities of some alloys, such as the nickel-iron alloy referred to above, by adding a third element, which further reduces the eddy current loss. The third element may be of such material noted by comparing the curves of Fig. 3 with those'of Fig.1. These curvesindicate only the relative valuesof the resistance and inductance as compared tothe zerocurrent values. The actual values of resistance and inductance would be much less for the con- 7 ductor of Fig.3 than for that ofFig. 1. Therelative inductances of the two conductors (assuming them to be of the same diameter and the loading on'each tobe of the same thickness) would be represented by curves and 2 in Fig. 4 in which 1 represents. the inductance of the conductor loaded with high permeability magnetic material and2 the inductance of an identical conductor loaded with low permeability (and incidentally high resistivity) material. The inductancecurves have been chosen for comparison rather than the resistance curves because abrupt changes in the inductances of different sections of cable are more productive of objectionable reflection effects than are resistance changes. These curves show that"'although the Zero current" inductance of the conductor surrounded by high permeability loading material may be only abouttwicethat 'of'the conductor loaded with the low permeability material, it may have aninductance proportionally much greater at large current densities. The same is true of the effective re-.

sistance.

Now if the thickness of loading material on the conductorhaving a low permeabilitymaterial is increased the inductance of the conductor increases at all current densities substantially in proportion tothe increase in the amount of magnetic material added. It is therefore possible, by using thlckerloading of low permeability material, toiobtain a 0011- ductor having the'inductance characteristic shown by curve 3'in Fig. 41. As shown by the curve. such a conductor has substantially the same inductance at zero current as does the conductor having high permeability loading material, but the variation in inductance with current variations is much less. The variation in resistance due to hysteresis and eddy current losses will also be less.

The fact that two sections of a conductor a have the same distributed inductance (meas ured at some particular value ofcurrent such as. for example, a vanishingly small current) does not necessarily mean that they have exactly the same transmission efiiciency. For this'reason it may be desirable to so proportionthe loadingon the terminal sections that the curve 3' of F ig. 4 intersects curve at'a point "to the right of, the :origin. However,

theadditional amount of lower permeability loading. material required to give-constant inductance is of the same'orde'r of; magnitude as thatrequired to glve constant transmls- S1011 650161163 In an ideal! cable intended for two-way transmission, the thickness and permeability oflthe loading material should vary continuously from the ends tothe center of the cable. This would be commercially impracticable, however, and in practice the main portion of the cablewould be uniformly loaded and the two end portions, probably a few hundred,

miles in length, would be dividedinto sections each of which would be uniformly loaded throughout its length, but indifferent degree ban the adjoining sections.

Fig, 5'shows along cable arranged for duplex telegraph operation andconstructed in accordance with this invention in which the thickness of the loading isprogressively de- 7 creased in four steps-fromthe maximum thickness at the terminal to the minimum thickness at thecentersection, and in'which I the permeability is increased in four steps from the mlmmum value at the terminal sectlonto the maximum value at the center sec-' tion. I On all sections the product of the thickness of the loading material anclthe effective permeability -is approximately the same and therefore the inductance-of the conductor is substantially uniform throughout.

Although'the'impedance variations in different portions of the cable are much less than they would be if the loading were not tapered f the different sections will have somewhat dif-.

ferent impedance characteristics and should bebalanced by correspondingly different sections of artificial line as shown by N N N 7 and N in Fig. 5. a;

The effective thickness of'the-loadingmate i i rial may be :variedin difierent ways. A singlelayer of tape of different thickness .on

"different sectionsmay be used. A tape of uniform thickness maybe used on all sections but the lay" increased on the central sections.

as'shown in Fig. 6. A plurality of layers of tape may beused on the terminal sections and the number of layers and/ or the-thickness reduced on the sections more remote from the terminal as shown in Fig. 7. This construe I tion is the most desirable from an efiiclency standpoint because the division of the loading material into several layers reduces the eddy current losses.

utilized, forinstance continuous lump loading, in which'sections of cable are uniformly loaded and intervening sections, short as committed, are left unloaded. In accordance with thisinvention the terminal portions may beuniformly loaded with low permeability Other ways of reducing the loading may be ared to the length of the waves to be trans- 3 material and the central-portion loaded on uniformly separated sections only with high permeability material. 7

What is claimed is:

' 1. A long inductively loaded signaling'conductor having a great overall attenuation in whichthe quantity per unit and the permeability of the loading material are different on different sections but such that the transmission eficiency is approximately the same for all sections.

,2. A long inductively loaded signaling conductor having a great overall attenuation in which thethickness and permeability of the loading material are different in different sections but such that the transmission efiiciencies of all sections are substantially the same. v

3. A long signaling conductor having great overall attenuation and substantially uniform diameter throughout, surrounded by magnetic loading material the magnetic characterlstics of which vary wlth the denslty of the magnetic flux induced therein, charac terized in this, that the quantity of magnetic material-per unit length of conductor is greater, but its permeability and variation of permeability with magnetizing force are less on a section adjacent a terminal of the conductor than on a section more remote from the terminal.

4. A loaded conductor as defined in claim 3 further characterizedin that the transmission efliciency of each section of the conductor is substantially the same astbat of every other section.

5. A long submarine cable havin great overall attenuation comprising a conductor surrounded by one or more helical wrappings of magnetic material, characterized in that the number of Wrappings are greater, but the permeability and variation of permeability with magnetizing force ofthe material is less on a section of said'cable adjacent an end thereof than on a section more remote from an end, but further characterized in that the permeability and resistivity of the loading material on any section are so regulated *ith respect to the number of layers that all seetions have substantially the same transmission efficiency V p 6. Along submarine cable having great overall attenuation comprising a conductor surrounded by one or more helical wrappings of magnetic-- material, characterized in'that the thickness of each wrapping and the number of wrappings are greater,,but the permeability and variation of permeability with magnetizing force of the material is less on a section of said cable adjacent an-end thereof than on a section more remote from the end, but further characterized in that the transmission efiiciency of any section of conductor is the same as for every other section.

7. A loaded transmission conductor having material on another poi blOll.

9. A long submarine cable having great overall attenuation. comprising conductor surrounded by one or more helical wrappings of magnetic material, characterized in that the number of wrappings are greater, but the permeability of the material is less on a sectionof said cable adjacent an end thereof than on a section more remote from an end, but further characterized in that the permeability and resistivity of the loading material on any section are so regulated with respect to the number of layers that all sections have substantially the same effective inductance.

10. A long submarine cable having great overall attenuation comprising a conductor surrounded by one or more helical wrappings of magnetic material, characterized in that the thickness of each wrapping and the number of wrappings are greater, but the permeability of the material is less on a section of said cable adjacent an end thereof than on a section more remote from the end, but further characterized in that the product of the thickness of each layer, the number of layers, and the permeability of the material for any section is the same as for every other section.

11. A submarine cable for communication purposes, the intermediate portion of which is continuously loaded with a given amount per unit length of magnetic material of predetermined permeability and the terminal portions of which are continuously loaded with a sufficiently greater amount per unit length of magnetic material having a permeability lower than that first mentioned, to maintain the energy losses and frequency distortion due to the cable within given limits based on the larger current flowing in the initial portion as compared to that flowing in the remainder of the cable. if

12. In the art of electrical communication over Wires, the method of reducing the attenuation and distortion effects in along communication circuit which consists in increas ing the inductance of the circuit by uniformly distributing along the principal and centrally located portion of the circuit, a given amount per unit length of magnetic material of a predetermined permeability, and uniformly distributing along the terminal portions of the circuit a different amount per unit length of magnetic material of a permeability different from that first mentioned.

18. In the art of electrical communication I over Wires, the method of reducing: the attenuation-and distortion effects in a long submarine cable which consists in increasing the 1 inductance of the circuit by uniformly dis tributing along the principal and intermedlate length of cable, agiven amount per unit length of magnetic material of a predetermined permeability, and uniformly distribv uting along the terminal portions of the cable a greater amount per unit length of magnetic materialof a permeability lower than that first mentioned.

14:. In the art of electrical communication over wires, the method of reducing attenuation and distortion effects 1n a long submarlne cable which consists in'mcreaslng the 1nductance of the cable by uniformly distributing along the principal and intermediate length of cable, a given amount per unit length of magnetic material of a predetermined permeability, and uniformly distrib-a uting along the terminal portions of the cable a sufiiciently greater amount per unit length of magnetic material of a permeability lower than that first mentioned, to maintain the attenuation and distortion efi'ects due to said cable within predetermined limits de-. pendent upon the larger amount of signaling current flowing in the terminal portions of o the cable compared to that flowing in the intermedi ate portion.

15. A submarine cable for communication purposes, the intermediate portion of which is loaded continuously with a given amount per unitlength of magnetic materialof a predetermined permeability and the terminal pore tions of which are loaded continuously with adiiferent amount per unit length of magnetic material of a permeability my name this 4th day of June, 1928. JOHN J. GILBERT.

7 different 7 from that first mentioned.

In witness whereof, I hereunto subscrib 

